Integrated circuits (ICs) are frequently manufactured with hundreds of millions of transistors. As the size of transistors shrinks, the number of transistors in any given area of an integrated circuit rises. These transistors form discrete components such as logic gates. For example, NAND, NOR, AND, OR and XOR gates are formed form COMS transistors. In turn, various logic gates may be combined to form more sophisticated structures such as adders, registers, or the like. Transistors and logic gates are frequently used in processors to interpret and implement instructions during execution of software programs. Similarly, embedded systems may have instructions residing in system memory, and a processor may access and execute those instructions using transistors or logic gates.
When transistors in an IC are formed, they are generally formed during the same fabrication process. However, imperfections in the fabrication process can lead to transistors fabrication in a single process exhibiting some variation on operating parameters. Likewise, transistors from different ICs may exhibit variations in operating parameters due to variations in the fabrication process. For example, transistors may have dopants applied to source and drain regions. However, the dopant application or concentrations may not be perfectly uniform cross transistors in a single chip, or across different chips. As another example, a gate oxide may be created by bringing oxygen into contact with the silicon surface under controlled heat and pressure conditions. The inconsistencies in oxygen concentration at the wafer surface during gate oxide formation may result in some slight inconsistencies in the thickness of different transistors' gate oxide, and by extension, cause inconsistencies in the operation of the transistors. In some instances, a high-k dielectric or oxynitride may be used to form a gate oxide, with those materials being deposited, for example, by chemical vapor deposition. The deposition of these materials may have slight inconsistencies, and may lead to irregularities in the gate oxide thickness and operating parameters of the transistors.
Another issue encountered with transistors is aging transistors during actual use. Dopant migration, contact electromigration, oxide breakdown, bias temperature instability or hot carrier injection into the gate oxide are only some types of defects that arise when transistors age under even normal use. Aging of transistors can result in a higher voltage required to turn on a transistor, or a slower turn-on time.